Solid-state neutron detector

ABSTRACT

A method for fabricating a neutron detector includes providing an epilayer wafer of Boron-10 enriched hexagonal boron nitride (h-10BN or h-BN or 10BN or BN) having a thickness (t), dicing or cutting the epilayer wafer into one or more BN strips having a width (W) and a length (L), and depositing a first metal contact on a first surface of at least one of the BN strip and a second metal contact on a second surface of the at least one BN strip. The neutron detector includes an electrically insulating submount, a BN epilayer of Boron-10 enriched hexagonal boron nitride (h-10BN or h-BN or 10BN or BN) placed on the insulating submount, a first metal contact deposited on a first surface of the BN epilayer, and a second metal contact deposited on a second surface of the BN epilayer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and is a divisional patentapplication of U.S. patent application Ser. No. 16/170,500 filed on Oct.25, 2018 and entitled “Solid-State Neutron Detector”, which is herebyincorporated by reference in its entirety.

STATEMENT OF FEDERALLY FUNDED RESEARCH

This invention was made with government support under grant/contractnumber DE-AR0000964 awarded by the Department of Energy, AdvancedResearch Projects Agency-Energy (ARPA-E). The government has certainrights in the invention.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to the field of solid-statedetectors and more specifically to solid-state neutron detectors and amethod of fabricating solid-state neutron detectors.

BACKGROUND OF THE INVENTION

Without limiting the scope of the invention, its background is describedin connection with neutron detectors.

Detection of neutrons is an exceedingly specific indicator of thepresence of fissile materials. As such, neutron detectors are deployedat ports of entry throughout the US for the purpose of detecting andinterdicting the movement of an illicit special nuclear material (SNM)or an actual nuclear device [1]. Neutron detectors are also anindispensable tool in geothermal and well-logging for the determinationof the formation properties, including the porosity and water (and/orH₂) content [2]. Similarly, neutron detectors are also useful forplanetary missions for remotely sensing the water (and/or H₂) content inthe shallow subsurface and/or to determine the surface compositions ofplanetary bodies [3]. When a fast neutron emitted from a neutrongenerator strikes a hydrogen nucleus of equal mass, which is present inpore water/oil, it thermalizes. Modern neutron logging tools commonlycount thermal and epithermal neutrons by employing pressurized ₂ ³He(He-3) gas tube detectors. To the first order, the thermal neutron countis inversely proportional to the hydrogen content (or the porosity) ofthe rocks. The most widely deployed neutron detectors are helium-3 gasdetectors. This is because He-3 has a very high thermal neutron capturecross-section of 5330 barn [4]. However, being a gas, He-3 detectors areinherently bulky. Moreover, its scarcity has had an extreme effect onits price in recent days. Other shortcomings of He-3 detectors are theneed of high pressurization (up to 20 atm for 2.25-inch diameter tubes),high voltage application (>1000 V) and slow response speed(˜milliseconds). These attributes prohibit flexibilities in detectordesign and form factors and also increasemeasurements/exploration/logging time and costs. Additionally, He-3 gasdetectors are most appropriate for operation below 175° C. For welllogging, the trend is moving into deep and slim wells where temperatureseasily exceed 250° C. For geothermal logging, the environmentalconditions are even more extreme where temperatures can be as high as500° C. Therefore, neutron detectors with enhanced capabilities ofoperating in extreme environments of high temperatures/mechanicalvibration/shock are highly desirable. Solid-state thermal neutrondetectors have recently been rapidly developed [5-22] for their obviousadvantages including independence from ³He gas, compactness, and lowvoltage operation. Until this date, the most effective solid-statedetector approach has been the micro-structured semiconductor neutrondetector (MSND), which has been reported extensively in recent years[5-16]. This type of indirect conversion detector is composed ofmicro-structured Si filled with either ¹⁰B or ⁶LiF. The detectionefficiency depends upon microstructure design, material choice, anddepth of the reacting material. The most efficient micro-structuredsemiconductor based thermal neutron detector that has ever been reportedconsists of a ¹⁰B filled Si microstructure with an efficiency of 48.5%[8, 9]. On the other hand, stacked ⁶LiF filled Si detectors [6, 7,11-13] with a certified detection efficiency of 30% have already beencommercialized. The theoretical and actual attained detectionefficiencies of these existing solid-state neutron detectors are limitedby the intrinsic material properties and device architectures employed.

In recent years, single crystalline hexagonal boron nitride (h-BN) widebandgap semiconductor has emerged as an attractive material for neutrondetector applications [23-31]. This is due to the fact that singlecrystal h-BN films (or epilayers) can be synthesized by epitaxial growthtechniques such as metal organic chemical vapor deposition (MOCVD)[23-30] and that the thermal neutron capture cross-section of Boron-10(¹⁰B) isotope is quite high (σ˜3840 barns=3.84×10⁻²¹ cm²) [4]. Becauseit is composed of low atomic number elements, B(5) and N(7), h-BN'sinteraction with gamma photons is extremely low, which gives rise to anexcellent gamma to neutron discrimination ratio below 10⁻⁶ [26, 28].

The element B exists as two main isotopes, ¹⁰B and ¹¹B in a naturalabundance of approximately 20% and 80% respectively [4]. It is only theisotope ¹⁰B that can interact with neutrons. FIG. 1 is a plot of neutroncapture cross sections as functions of the kinetic energy of neutronsfor He-3 (green upper plot), B-10 (orange middle plot), and Li-6 (purplelower plot). FIG. 1 shows that ¹⁰B (orange middle plot) has a largecapture cross section (a) of about 3840 barn=3.84×10⁻²¹ cm²) for thermalneutrons (neutrons with an energy=25 meV), which is only slightlysmaller than a value of σ˜5330 barns for He-3 gas atoms (green upperplot) [4]. [adopted from MITOpenCourseWare—https://ocw.mit.edu/courses/nuclear-engineering/22-106-neutron-interactions-and-applications-spring-2010/lecture-notes/MIT22_106S10_lec07.pdf.(Slide 27)]. However, as a semiconductor, the density of atoms which caninteract with thermal neutrons in 100% ¹⁰B-enriched BN (¹⁰BN) isN(¹⁰B)=5.5×10²²/cm³, which is about 550 times higher than that in He-3gas pressurized at 4 atm. This provides an absorption coefficient forthermal neutrons in ¹⁰BN of α=Nσ=5.5×10²²×3.84×10⁻²¹=211.2 cm⁻¹ and anabsorption length of λ=α⁻¹=47.3 μm [28-30]. This thickness is negligiblysmall compared to the dimensions of He-3 gas detectors which typicallyhave diameters in inches.

Earlier h-BN neutron detectors, such as those disclosed in U.S. Pat. No.9,093,581 which is hereby incorporated by reference in its entirety,were based on a metal-semiconductor-metal (MSM) architecture withmicro-strip interdigital fingers fabricated on h-BN epilayers of severalmicrons in thickness [24-27]. The photolithography technique was used topattern the interdigital fingers on the surface of h-BN epilayers.Pattern transfer was accomplished using inductively-coupled plasma (ICP)dry etching. The patterns were etched all the way to the sapphiresubstrate. Metal contacts were deposited by e-beam evaporation. Thedetection efficiencies of these devices were limited to a few percent atthe best since this type of MSM device architecture involves dry etchingand is limited to the fabrication of very thin h-BN detectors. Moreover,a fraction of the detection area must be removed by dry etching in theMSM detectors, which sacrifices the overall detection sensitivity.Furthermore, dry etching also induces surface damages, which increasessurface recombination and reduces the charge collection efficiency.Therefore, these MSM detectors are only suitable for initial conceptualdemonstration. Accordingly, a need remains in the art for solid-stateneutron detectors.

SUMMARY OF THE INVENTION

The present invention relates to the design and fabrication of asolid-state neutron detector which is capable to provide high quantumefficiency and detection sensitivity and to operate in harshenvironments (including high temperature and high pressure). Thesedetectors are fabricated from single crystal boron nitride (BN)semiconductors in thin film form with large thicknesses to support anintrinsic detection efficiency approaching 100%. High detectionsensitivity is accomplished via lateral conduction and detector arrayconfigurations to provide a high charge collection efficiency and largedetection area. These detectors are able to withstand extremely hightemperatures due to the fact that these materials possess high thermalconductivity and large energy bandgap and that they are synthesized atvery high temperatures.

These BN neutron detectors are more durable and require much lowervoltages and power consumption and no pressurization compared to theHe-3 gas detectors, thereby providing significant reduction in size andweight, more versatile form factors, faster response speed, higherreliability, and lower costs for fabrication/operation/maintenance overthose of He-3 gas detectors for many applications, including in theareas of detection of nuclear materials, geothermal and well logging,and planetary missions.

One embodiment of the present invention provides a method forfabricating a neutron detector by providing an epilayer wafer ofBoron-10 enriched hexagonal boron nitride (h-¹⁰BN or h-BN or ¹⁰BN or BN)having a thickness (t), dicing or cutting the epilayer wafer into one ormore BN strips having a width (W) and a length (L), and depositing afirst metal contact on a first surface of at least one of the BN stripsand a second metal contact on a second surface of the at least one BNstrip.

In one aspect, the method includes connecting the first metal contactand the second metal contact to a measurement circuit. In anotheraspect, the method includes measuring a plurality of neutrons using theneutron detector. In another aspect, the method includes using theneutron detector to detect nuclear materials, to perform geothermaland/or well logging, or perform planetary missions. In another aspect,the method includes placing the at least one BN strip onto anelectrically insulating submount prior to depositing the first metalcontact and the second metal contact; and wherein the first surfacecomprises a side of the at least on BN strip and the second surfacecomprises an opposite side of the at least one BN strip such that theneutron detector comprises a lateral oriented neutron detector. Inanother aspect, the first metal contact and the second metal contacteach overlaps a top edge of the at least one BN strip. In anotheraspect, the method includes repeating the placing step and depositingstep such that a plurality of BN strips are placed end to end onto theelectrically insulating submount in a series configuration. In anotheraspect, the method includes repeating the placing step and depositingstep such that a plurality of the neutron detectors are placed onto theelectrically insulating submount in a parallel configuration. In anotheraspect, depositing the first metal contact on the first surface of theat least one BN strip and the second metal contact on the second surfaceof the at least one BN strip includes: depositing a first metal contactonto an electrically insulating submount; placing the first surfacecomprising a bottom surface of the at least one BN strip onto the firstmetal contact; and depositing the second metal contact onto the secondsurface comprising a top surface of the at least one BN strip such thatthe neutron detector comprises a vertical oriented neutron detector. Inanother aspect, depositing the first metal contact on the first surfaceof the at least one BN strip and the second metal contact on the secondsurface of the at least one BN strip is preformed using an electron-beamevaporation, thermal evaporation, electroplating or pasting process. Inanother aspect, the one or more BN strips comprise at least two strips,each strip having the width (W) and at least two lengths (L₁, L₂). Inanother aspect, the thickness of the epilayer wafer comprises severalmicrons to greater than 300 μm. In another aspect, providing theepilayer wafer includes: providing a substrate; growing the epilayerwafer on the substrate; and removing the epilayer wafer from thesubstrate. In another aspect, removing the epilayer wafer from thesubstrate includes cooling the epilayer wafer and the substrate aftercrystal growth such that a difference in thermal expansion coefficientsbetween the epilayer wafer and the substrate automatically separates theepilayer wafer from the substrate. In another aspect, the epilayer waferis flexible. In another aspect, the neutron detector has one or morecharacteristics comprising an operating voltage that can be less thanabout 500 V, an operating temperature between about −200 and 500 C, agamma rejection ratio of about 1×10⁻⁴ or better, or an energy resolutionof about 3% or better.

Another embodiment of the present invention provides a neutron detectorthat includes an electrically insulating submount, a BN epilayer ofBoron-10 enriched hexagonal boron nitride (h-¹⁰BN or h-BN or ¹⁰BN or BN)placed on the insulating submount, a first metal contact deposited on afirst surface of the BN epilayer, and a second metal contact depositedon a second surface of the BN epilayer.

In one aspect, a measurement circuit is connected to the first metalcontact and the second metal contact. In another aspect, the firstsurface comprises a side of the BN epilayer and the second surfacecomprises an opposite side of the BN epilayer such that the neutrondetector comprises a lateral oriented neutron detector. In anotheraspect, the first metal contact and the second metal contact eachoverlaps a top edge of the BN epilayer. In another aspect, the BNepilayer comprises a plurality of BN strips placed end to end onto theelectrically insulating submount in a series configuration, and each BNstrip having the first metal contact and the second metal contact. Inanother aspect, the BN epilayer comprises a plurality of the BN stripsplaced onto the electrically insulating submount in a parallelconfiguration, and each BN strip having the first metal contact and thesecond metal contact. In another aspect, the first surface comprising abottom surface of the BN epilayer, the second metal contact is disposedonto the second surface comprising a top surface of the BN epilayer, andthe neutron detector comprises a vertical oriented neutron detector. Inanother aspect, the first metal contact and the second metal contact aredeposited using an electron-beam evaporation, thermal evaporation,electroplating or pasting process. In another aspect, the BN epilayercomprises at least two BN strips, each BN strip having a width (W) andat least two lengths (L₁, L₂). In another aspect, a thickness of the BNepilayer comprises from several microns to greater than 300 μm. Inanother aspect, the neutron detector has one or more characteristicscomprising an operating voltage that can be less than about 500 V, anoperating temperature between about −200 and 500 C, a gamma rejectionratio of 1×10⁻⁴ or better, or an energy resolution of about 3% orbetter. In another aspect, the neutron detector is integrated into anuclear material detection device, a geothermal and/or well loggingdevice, or a planetary mission device

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the features and advantages of thepresent invention, reference is now made to the detailed description ofthe invention along with the accompanying figures and in which:

FIG. 1 is a plot of neutron capture cross sections as functions of thekinetic energy of neutrons for He-3, B-10, and L₁-6.

FIG. 2 is a plot of the theoretical thermal neutrons detectionefficiency of 100% 10B-enriched h-BN with respect to film thickness.

FIG. 3 is a photo of a 4-inch freestanding 10BN wafer of 50 μm inthickness produced in accordance with one embodiment of the presentinvention.

FIG. 4A is a schematic illustration of crystal structure of hexagonal BNin accordance with one embodiment of the present invention.

FIG. 4B is a photo of a piece of free-standing 100% B-10 enriched BN(10BN) epilayer showing its flexibility in accordance with oneembodiment of the present invention.

FIG. 5A is a schematic diagram of a photoconductive-type vertical ¹⁰BNneutron detector in accordance with one embodiment of the presentinvention.

FIG. 5B is a photo of a packaged photoconductive-type vertical ¹⁰BNneutron detector in accordance with one embodiment of the presentinvention.

FIGS. 5C and 5D show plots of pulse height spectra of a 50 μm thick ¹⁰BNdetector in response to a ²⁵²Cf neutron source moderated by a highdensity polyethylene (HDPE) block (red curve) and to 0.662 MeVgamma-photons produced by ¹³⁷Cs decay (green curve).

FIGS. 6A and 6B are schematic diagrams a cross-sectional view and sideview, respectively, of a photoconductive-type lateral ¹⁰BN neutrondetector with a width of W and length L in accordance with oneembodiment of the present invention.

FIG. 7 is a photo of a fabricated photoconductive-type lateral ¹⁰BNneutron detector with a width of W and length L in accordance with oneembodiment of the present invention.

FIG. 8 is a plot of the photocurrent characteristics of a lateral ¹⁰BNneutron detector in accordance with one embodiment of the presentinvention.

FIGS. 9A and 9B are schematic diagrams illustrating the attainment ofdetector strips with a width W from 4-inch and 6-inch BN wafers,respectively, to realize large effective detection area and detectionsensitivity in accordance with one embodiment of the present invention.

FIGS. 10A-10C are cross-sectional, side and top views, respectively ofmany lateral ¹⁰BN neutron detectors of the same width but differentlengths to obtain a large detector size and provide a high detectionsensitivity in accordance with one embodiment of the present invention.

FIG. 11 is a flowchart of a method for fabricating a single crystalepilayer wafer in accordance with one embodiment of the presentinvention.

FIG. 12 is a flowchart of a method for fabricating a neutron detector inaccordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

While the making and using of various embodiments of the presentinvention are discussed in detail below, it should be appreciated thatthe present invention provides many applicable inventive concepts thatcan be embodied in a wide variety of specific contexts. The specificembodiments discussed herein are merely illustrative of specific ways tomake and use the invention and do not delimit the scope of theinvention.

To facilitate the understanding of this invention, a number of terms aredefined below. Terms defined herein have meanings as commonly understoodby a person of ordinary skill in the areas relevant to the presentinvention. Terms such as “a”, “an” and “the” are not intended to referto only a singular entity, but include the general class of which aspecific example may be used for illustration. The terminology herein isused to describe specific embodiments of the invention, but their usagedoes not limit the invention, except as outlined in the claims.

The present invention is related to the design and fabrication of ¹⁰BNsolid-state detectors. Compared to He-3 tube detectors, BN detectorswill provide significant performance enhancements, as summarized inTable 1, which include increased detection efficiency and operatingtemperature and ruggedness, reduced size and weight, no pressurization,lower operating voltage and power consumption, and larger/faster signalsin extreme environments, and consequently decrease theexploration/logging time and the costs of operation/maintenance andpotentially enable logging tools to operate in harsh environments whereHe-3 detectors are not capable to operate. In particular, this inventionprovides ¹⁰BN solid-state detectors with high detection efficiencies andsensitivities.

TABLE 1 Characteristics comparison between He-3 gas tube detectors and¹⁰BN detectors He-3 gas detector ¹⁰BN semiconductor detector Nuclear³He + ¹n = ¹H (573 ¹⁰ ₅B + ¹ ₀n = ⁷ ₃Li* (0.84 MeV) + reaction keV) + ³H(191 keV) ⁴ ₂α* (1.47 MeV), 94% ¹⁰ ₅B + ¹ ₀n = ⁷ ₃Li (1.015 MeV) + ⁴ ₂α(1.777 MeV), 6% Li, α → N (e⁻) + N (h⁺), N ~ 10⁵ Intrinsic 77% (ϕ = 1″ @4 atm) 95% (@140 μm thickness) efficiency for (0.025 eV) thermalneutrons Intrinsic 20% (ϕ = 1″ @ 4 atm) 63% (@200 μm thickness)efficiency for (0.4 eV) epithermal neutrons Method for Increase gas tubeIncrease the thickness and increasing diameter, length, total detectionarea of sensitivity pressure ¹⁰BN detector chip Response ~1 ms ~1 nsspeed Operating >1000 <500 (<100) voltage (V) Typical −50 to +150 −200to +500 operating temperature, ° C. Gamma ~1 × 10⁻⁴ ~1 × 10⁻⁴ (~1 ×10⁻⁶) rejection ratio Vibration Movement causes Semiconductor packagesresponse spurious pulses immune to vibration Energy 6% 3% resolutionActive 300-500 1 volume ratio (depending on the for gas tube diameterequivalent and pressure) sensitivity Cost for $2,400 (ϕ = 1″; ~$240 (2 ×2″ wafers, equivalent L = 25 cm @ 4 atm) 80 μm thick) sensitivity Notes:Both He-3 & BN detectors are nearly insensitive to fast neutrons(neutrons with E > 0.1 MeV). Scintillator detectors are sensitive toboth thermal neutrons and gamma photons and require additional methodsto discriminate thermal neutrons from gamma photons, which are notpreferred in modern neutron logging tools and may be more suitable forgamma-ray logging.

The thermal neutron absorption probability (P) or the theoreticaldetection efficiency (η) of ¹⁰BN detectors as a function the detector'slayer thickness, t, can be expressed as [28, 29]

η=P(t)=1−e ^(−t/λ),  (1)

where λ=47.3 μm (thermal neutron absorption length). Eq. (1) is plottedin FIG. 2, which shows that the efficiency of ¹⁰BN detectors can beincreased by simply increasing the layer thickness. This is in sharpcontrast to He-3 gas detectors in which the enhancement in the detectionsensitivity is achieved through increasing the gas tube diameter, lengthand gas pressure. As indicated in FIG. 2, BN detectors with a layerthickness of 3λ (˜140 μm) can attain a theoretical detection efficiencyof 95%, whereas BN detectors with a layer thickness of t>300 μm canattain a theoretical detection efficiency approaching 100%.

As shown in FIG. 3, a freestanding and single crystal of 100% Boron-10enriched hexagonal boron nitride (abbreviated as h-¹⁰BN or h-BN or ¹⁰BNor BN hereafter) epilayer wafers of 4-inches in diameter with largethicknesses (>30 μm) has been successfully produced. Due to the uniquelayered structure of h-BN (FIG. 4A) and the difference in thermalexpansion coefficients between h-BN and substrate, h-BN epilayers with asufficient thickness automatically separate from substrates duringcooling down after growth, forming freestanding h-BN wafers. Asdemonstrated in FIG. 4B, freestanding BN epilayers are quite flexible.Photoconductive-type of vertical detectors 500 such as that shown inFIGS. 5A and 5B can be easily constructed from these freestandingwafers. The fabrication processes include two steps: dicing (or cutting)¹⁰BN wafers into desired shapes and dimensions 502 followed by topcontact 504 and bottom contact 506 deposition (e.g., Ni/Au (15 nm/20nm)). The top and bottom contacts 502, 504 are then connected to ameasurement circuit 508.

The vertically oriented neutron detector 500 includes a BN epilayer 502having a first surface (bottom surface) and a second surface (topsurface), a first metal contact 506 disposed on the first surface(bottom surface) of the BN epilayer 502, and a second metal contact 504deposited on a second surface (top surface) of the BN epilayer 502. TheBN epilayer 502 is Boron-10 enriched hexagonal boron nitride having athickness (t), a width (W) and a length (L). A measurement circuit 508is connected to the first metal contact 506 and the second metal contact504.

The first metal contact 506 and the second metal contact 504 can bedeposited using an electron-beam evaporation, thermal evaporation,electroplating or pasting process. The thickness of the BN epilayer 502can be from about a few microns to greater than 300 μm. The neutrondetector 500 has one or more characteristics, such as an operatingvoltage less than about 500 V, an operating temperature between about−200 and 500 C, a gamma rejection ratio of about 1×10⁻⁶, or an energyresolution of about 3%. In some embodiments, the operating voltage lessthan about 100 V or the gamma rejection ratio is better than about1×10⁻⁴.

As shown in FIGS. 5C and 5D, neutron detectors fabricated from 50 μmthick ¹⁰BN epilayers possess a record high thermal neutron detectionefficiency among solid-state neutron detectors (range from 53%-58%) anda gamma rejection ratio (GRR) of better than 10⁻⁶ [28-30]. In FIG. 5C,the green left most curve plots the counts to channel with a ¹³⁷Cs gammasource, the blue center-curve plots the counts to channel without anysource, and the red right most curve plots the counts to channel with a²⁵²Cf neutron source. In FIG. 5D, the blue left most curve plots thecounts to channel without any source, and the red right most curve plotsthe counts to channel with a ²⁵²Cf neutron source

The operating principle of BN neutron detectors is based on the factthat absorption of a neutron by a ¹⁰B atom induces the following nuclearreaction inside ¹⁰BN [4],

₅ ¹⁰B+₀ ¹ n= ₃ ⁷Li^(*)(0.84 MeV)+₂ ⁴α^(*)(1.47 MeV) [94% excitedstate]  (2a)

₅ ¹⁰B+₀ ¹ n= ₃ ⁷Li(1.015 MeV)+₂ ⁴α(1.777 MeV) [6% ground state]  (2b)

Li,α→N(e ⁻)+N(h ⁺) N˜10⁵  (3)

The detection of neutrons by a BN detector is accomplished by twosequential processes. The first is the neutron absorption described byEq. (2) in which the nuclear reaction creates L₁ and a daughterparticles with large kinetic energies. The second process of Eq. (3) isthe charge carrier generation by Li and α particles and the subsequentcollection of charge carriers [electrons (e⁻) and holes (h⁺)]. Incontrast to ⁶LiF [6, 7, 11-13] or ¹⁰B [8, 9, 14-16] filledmicro-structured semiconductor neutron detectors, the two sequentialprocesses described by Eqs. (2) and (3) occur in the same BN layer.Therefore, BN detectors are considered as direct conversion neutrondetectors and are capable to provide high charge collection efficiencyand hence high detection efficiency for thermal neutrons. On the otherhand, the unique properties that set single crystal BN material apartfrom other direct conversion neutron detectors based on amorphous B₄C[17], gadolinium complexes [18], pyrolytic and polycrystalline BN, andalpha rhombahedral boron complexes [19-22] include: (1) hexagonal BN hasa simple crystal structure, which allows for the attainment of singlecrystalline thin films by epitaxial growth techniques such as MOCVD and(2) materials with single crystalline structure contain few charge trapsand allow a rapid sweep-out of the electrons and holes generated by thenuclear reaction and high charge collection efficiency. Moreover, due tothe high thermal conductivity and high melting point of BN, BN detectorsare able to withstand extremely high temperatures.

Because the neutron flux in the relevant environments is usually low,high sensitivity detectors are desired for practical applications. Thedetection sensitivity of a detector, or the count rate (C_(R)) detectedby a detector, is proportional to its detection efficiency (η) and area(A), i.e.,

C _(R) ˜ηA.  (4)

Therefore, scaling up the detector size while maintaining a highdetection efficiency is necessary to enhance the detection sensitivity.For the vertical device architecture shown in FIG. 5A, due to the largecontact area between the metal contacts and the BN detector material,there are several technical challenges to scale up to large sizedetectors. These include: (1) The dark current (or leakage current)tends to increase with an increase of the detector area for a fixed biasvoltage; (2) The capacitance also increases with increasing the detectorarea; and (3) The effect of surface recombination increases withincreasing the contact area. It was shown that the equivalent noise ofthe measurement circuit increases linearly with increasing the leakagecurrent and quadratically with increasing the capacitance [32]. Thepresence of a large noise in the measurement circuit will have adetrimental effect on detecting the actual signal, whereas an increasein the number of surface charge carrier traps will decrease theefficiency of charge collection described by Eq. (3). All of these willhave consequences on the overall efficiency of the detector.

It is also critically important to understand the charge collectionprocess described by Eq. (3). Most of the neutron-generated chargecarriers, electrons (e⁻) and holes (h⁺) inside a detector can becollected by the electrodes when the condition of the recombination time(τ)≥the transit time (τ_(t)) is satisfied, i.e., μτ≥W²/Vτ_(t)=W/μE,E=V/W, or equivalently

$\begin{matrix}{{E \geq \frac{W}{\mu\tau}},\mspace{14mu} {or}} & \left( {5a} \right) \\{{V \geq \frac{W^{2}}{\mu\tau}},\mspace{14mu} {or}} & \left( {5b} \right) \\{{{W \leq \sqrt{V\; {\mu\tau}}},}\;} & \left( {5c} \right)\end{matrix}$

where μ is the charge carrier mobility, τ is the mean lifetime of chargecarriers, W is the carrier transit distance, and E (V) is the appliedelectric field (bias voltage). Note that the carrier transit distance(W) is equal to the thickness (t) of the detector for vertical orienteddetectors. Eq. (5) implies that the required bias voltage to achieve thesame charge collection efficiency is inversely proportional to themobility-lifetime product (μτ) of the detector material, whereas thequantity of μτ is strongly influenced by the overall material quality.As illustrated in FIG. 4A, it is well known that hexagonal boron nitrideis a layer structured material, in which very different bonding, i.e.,strong covalent bonding within the basal planes and weak bonding betweenplanes, leads to anisotropy in electronic transport properties. It wasshown that the mobility-lifetime products (μτ) for both electrons andholes can be three orders of magnitude larger in the lateral direction(within the basal planes) than in the vertical direction (betweenplanes) [33]. As such, the lateral device architecture of this inventionshown in FIGS. 6A-6B which utilizes the lateral transport properties ispreferred over that of the vertical devices (such as that shown in FIG.5A). According to Eq. (5), for the same carrier transit distance, thelateral detectors in principle would require 1000 times lower biasvoltage than the vertical detectors to attain a similar detectionefficiency. Furthermore, compared to the vertical devices shown in FIG.5A, the surface recombination effects are eliminated in lateraldetectors shown in FIGS. 6A-6B due the minimal contacts between theelectrodes and the detector's surface. The present invention thereforeprovides high efficiency and high sensitivity BN neutron detectors viathe formation of lateral detectors.

As shown in FIG. 4B, freestanding BN epilayers are flexible. A lateraldetector 600 can be formed by simply dicing or cutting a BN wafer intostrips and placing a BN strip 602 (with a width W and length L) onto anelectrically insulating submount 604 followed by metal contactdeposition covering the strip edges 608, as illustrated in FIGS. 6A-6B.The electrical contacts 606 and 608 on BN films and the electrodes mustbe sufficiently electrically conducting so that the free carriergenerated by nuclear reaction is not blocked from flowing into theexternal measurement circuit. Hence, low-resistance contacts are needed.This will ensure that essentially all of the electric field applied willbe across the BN detectors, as needed to sweep out the free chargecarriers. The metal contact type used in FIG. 5A provides just oneworking example. Many others metals may also be suitable to serve as themetal contacts on BN detectors. The lateral detector 600 is thenconnected to a measurement circuit 506. FIG. 7 shows photo of a lateraldetector 600 fabricated from a freestanding h-10BN of the presentinvention as described in FIGS. 6A-6B in which the insulating submount604 is sapphire. Other materials can be used for the insulating submount604.

In the lateral detector 600 of FIGS. 6A-6B, the desired width (W) isdetermined by the relation of Eq. (5) to ensure charge carriercollection. In the lateral direction, it can be assumed that the μτproducts for electrons and holes are identical due to the fact that theeffective masses of electrons and holes in single sheet h-BN areidentical (around 0.54 m_(o)) [34]. A higher material quality translatesto a large value of μτ product, which allows for a wider W and hencehigher detection sensitivity under a fixed bias voltage. Assuming thatthe μτ products for electrons and holes are identical, the μτ productcan be determined from the I-V characteristics under photoexcitationbased on a modified Many's equation for lateral charge transport [35],

$\begin{matrix}{{I(V)} = {I_{0}{\frac{2{\mu\tau}\; V}{W^{2}}\left\lbrack {1 - {\frac{{\mu\tau}\; V}{W^{2}}\left( {1 - e^{- \frac{W^{2}}{{\mu\tau}\; V}}} \right)}} \right\rbrack}}} & (6)\end{matrix}$

where V is the applied voltage and W is the detector's width (or carriertransit distance). FIG. 8 shows the I-V characteristics underphotoexcitation for one of the BN lateral detectors of this inventionand the fitting between experimental data and the Many's Equation yieldsa value of μτ product of about 1×10⁻⁴ cm²/V. This means that at a biasvoltage of 400 V the width of the detector must be made to satisfy thecondition of W²≤Vμτ (=400 V×10⁻⁴ cm²/V=0.04 cm² or W≤0.2 cm) in order toensure a sufficient charge carrier collection. Since the development offreestanding thick h-¹⁰BN epilayers is in an early stage, we anticipatesignificant enhancement in material quality as well as in the μτproducts. To put thing into a perspective, roughly an order enhancementin μτ value would allow the detector width W to increase to 6 mm(W²≤V·μτ=400 V×10⁻³ cm²/V=0.4 cm², W≤0.63 cm), which effectively alsoincreases the detection area and hence the detection sensitivity by 3times.

The theoretical detection efficiency (η) of these lateral detectors isdetermined by the detector's layer thickness (t) according to Eq. (1),whereas the overall detection sensitivity depends on both the detectionefficiency (η) and the device area (A=W·L) according to Eq. (4). It isimportant to point out that there is no limit in the sensitivity of adetector since it can be increased through increases in both the width(W) and length (L) of the detector. For BN wafers with a fixed μτproduct and hence a predetermined W based on Eq. (5), in order toincrease the overall detection sensitivity, one just needs to increasethe length (L) of the detector. Therefore, it is much easier to scale upthe detection size for lateral detectors of the present inventioncompared to the vertical detectors [22-24]. Very high sensitivitydetectors can be obtained using very long lateral (or strip detectors).

For benchmarking purpose, the commercially available micro-structuredsemiconductor neutron detector (MSND) from Radiation DetectionTechnologies, Inc. was constructed from a 2×2 detector array with aspecified a detection efficiency of 30% for thermal neutrons and with aneffective device area of 4 cm², which provides a relative detectionsensitivity of 1.2 (C_(R)˜ηA=4 cm²×30%). A BN lateral detectorincorporating an epilayer with a thickness greater than 140 μm canpotentially provide a detection efficiency approaching 90%. To attain asimilar detection sensitivity of 1.2 as the MSND, BN lateral detectorswith μτ product of 1×10⁻⁴ cm²/V and a width of W=2 mm need to have alength of L=6.6 cm in order to provide a comparable detectionsensitivity of 1.2 as the MSND. On other hand, BN lateral detectors witha μτ product of 1×10⁻³ cm²/V and a width of W=6 mm only need a length ofL=2.2 cm in order to provide a comparable detection sensitivity of 1.2as the MSND.

The laterally oriented neutron detector 600 includes an electricallyinsulating submount 604, a BN epilayer 602 placed on the insulatingsurmount 604, a first metal contact 606 deposited on a first surface(side) of the BN epilayer 602, and a second metal contact 608 depositedon a second surface (opposite side) of the BN epilayer 602. The BNepilayer 602 is Boron-10 enriched hexagonal boron nitride having athickness (t), a width (W) and a length (L). A measurement circuit 506is connected to the first metal contact 606 and the second metal contact608. The first metal contact 606 and the second metal contact 608 eachoverlaps a top edge 610 of the BN epilayer 602.

The first metal contact 606 and the second metal contact 608 can bedeposited using an electron-beam evaporation, thermal evaporation,electroplating or pasting process. The thickness of the BN epilayer 602can be from about several microns to greater than 300 μm. The neutrondetector 600 can have one or more characteristics, such as an operatingvoltage that can be less than about 500 V, an operating temperaturebetween about −200 and 500 C, a gamma rejection ratio of about 1×10⁻⁶,or an energy resolution of about 3%. In some embodiments, the operatingvoltage less than about 100 V or the gamma rejection ratio is betterthan about 1×10⁻⁴. The neutron detector 600 can be integrated into anuclear material detection device, a geothermal and/or well loggingdevice, or a planetary mission device.

As shown schematically in FIGS. 9A-9B, the longest detector 902 that canbe fabricated from 4-inch wafers 904 is about 10 cm, whereas the longestdetector 906 that can be fabricated from 6-inch wafers 908 is about 15cm. However, a single wafer can be diced into many strips with the samewidth and different lengths as schematically illustrated in FIG. 9B. Onecould continue to extend the length of the detector by attach manystrips together to achieve a very large detection area and highdetection sensitivity.

FIGS. 10A-10C are cross-sectional, side and top views, respectively ofmany lateral ¹⁰BN neutron detectors 600 ₁₋₃ of the same width W butdifferent lengths L₁, L₂, L₃ to obtain a large detector size and providea high detection sensitivity in accordance with one embodiment of thepresent invention. The ¹⁰BN neutron detector 1000 is made up of two ormore individual ¹⁰BN neutron detectors 600. Note that the number ofindividual ¹⁰BN neutron detectors used for the large detector 1000 canbe more or less than the example shown in FIGS. 10A-10C. Moreover, thelengths of all the detectors 600 ₁₋₃ can be the same.

The first laterally oriented neutron detector 600 ₁ includes anelectrically insulating submount 604, a BN epilayer 602 ₁ placed on theinsulating submount 604, a first metal contact 606 ₁ deposited on afirst surface (side) of the BN epilayer 602 ₁, and a second metalcontact 608 ₁ deposited on a second surface (opposite side) of the BNepilayer 602 ₁. The BN epilayer 602 ₁ is Boron-10 enriched hexagonalboron nitride having a thickness (t), a width (W) and a length (L₁). Thefirst metal contact 606 ₁ and the second metal contact 608 ₁ eachoverlaps a top edge 610 of the BN epilayer 602 ₁.

The second laterally oriented neutron detector 600 ₂ includes anelectrically insulating submount 604, a BN epilayer 602 ₂ placed on theinsulating submount 604, a first metal contact 606 ₂ deposited on afirst surface (side) of the BN epilayer 602 ₂, and a second metalcontact 608 ₂ deposited on a second surface (opposite side) of the BNepilayer 602 ₂. The BN epilayer 602 ₂ is Boron-10 enriched hexagonalboron nitride having a thickness (t), a width (W) and a length (L₂). Thefirst metal contact 606 ₂ and the second metal contact 608 ₂ eachoverlaps a top edge 610 of the BN epilayer 602 ₂.

The third laterally oriented neutron detector 600 ₃ includes anelectrically insulating submount 604, a BN epilayer 602 ₃ placed on theinsulating submount 604, a first metal contact 606 ₃ deposited on afirst surface (side) of the BN epilayer 602 ₃, and a second metalcontact 608 ₃ deposited on a second surface (opposite side) of the BNepilayer 602 ₃. The BN epilayer 602 ₃ is Boron-10 enriched hexagonalboron nitride having a thickness (t), a width (W) and a length (L₃). Thefirst metal contact 606 ₃ and the second metal contact 608 ₃ eachoverlaps a top edge 610 of the BN epilayer 602 ₃.

A measurement circuit 506 is connected to the first metal contacts 606₁₋₃ and the second metal contacts 608 ₁₋₃. The first metal contacts 606₁₋₃ and the second metal contacts 608 ₁₋₃ can be deposited using anelectron-beam evaporation, thermal evaporation, electroplating orpasting process. The thickness of the BN epilayers 602 ₁₋₃ can be fromabout a few microns to greater than 300 μm. The neutron detector 1000can have one or more characteristics, such as an operating voltage lessthan about 500 V, an operating temperature between about −200 and 500 C,a gamma rejection ratio of about 1×10⁻⁶, or an energy resolution ofabout 3%. In some embodiments, the operating voltage less than about 100V or the gamma rejection ratio is better than about 1×10⁻⁴.

In the scenarios where shorter detectors are desired, an alternativeapproach is to connect multiple lateral detectors in parallel so thatthe total detection sensitivity (or count rate) of the detector array isobtained by summing up signals from all detectors in the parallelcircuit. In such a case, the large ¹⁰BN neutron detector would be madeup of two or more individual ¹⁰BN neutron detectors 600 placed onto theelectrically insulating submount in a parallel configuration.

FIG. 11 is a flowchart of a method 1100 for fabricating an epilayerwafer of Boron-10 enriched hexagonal boron nitride in accordance withone embodiment of the present invention. A substrate is provided inblock 1102, the epilayer wafer of Boron-10 enriched hexagonal boronnitride is grown on the substrate in block 1104, and the epilayer waferof Boron-10 enriched hexagonal boron nitride is removed from thesubstrate in block 1106. The epilayer wafer of Boron-10 enrichedhexagonal boron nitride can be removed from the substrate by cooling theepilayer wafer of Boron-10 enriched hexagonal boron nitride and thesubstrate such that a difference in thermal expansion coefficientsbetween the epilayer wafer of Boron-10 enriched hexagonal boron nitrideand the substrate automatically separates the epilayer wafer of Boron-10enriched hexagonal boron nitride from the substrate. Note that theepilayer wafer of Boron-10 enriched hexagonal boron nitride is flexible.

FIG. 11 is a flowchart of a method 1200 for fabricating a neutrondetector in accordance with one embodiment of the present invention. Anepilayer wafer of Boron-10 enriched hexagonal boron nitride having athickness (t) is provided in block 1202. The epilayer wafer is diced orcut into one or more BN strips having a width (W) and a length (L) inblock 1204. A first metal contact is deposited on a first surface of theat least one BN strip and a second metal contact is deposited on asecond surface of the at least one BN strip in block 1206.

Optional steps include connecting the first metal contact and the secondmetal contact to a measurement circuit in block 1208, measuring aplurality of neutrons using the neutron detector in block 1210, andusing the neutron detector to detect nuclear materials, to performgeothermal and/or well logging, or perform planetary missions in block1212.

The method 1200 can be used to fabricate a lateral oriented neutrondetector by placing the at least one BN strip onto an electricallyinsulating submount prior to depositing the first metal contact and thesecond metal contact. The first surface comprises a side of the at leastone BN strip and the second surface comprises an opposite side of the atleast one BN strip. The first metal contact and the second metal contactcan each overlaps a top edge of the at least one BN strip.

The method 1200 can be used to fabricate a long neutron detector byrepeating the placing step and depositing step such that a plurality ofBN strips are placed end to end onto the electrically insulatingsubmount in a series configuration. Each BN strip has the width (W) andat least two lengths (L₁, L₂). Alternatively, each BN strip can have thesame length.

The method 1200 can be used to fabricate a short neutron detector byrepeating the placing step and depositing step such that a plurality ofthe neutron detectors are placed onto the electrically insulatingsubmount in a parallel configuration.

The method 1200 can be used to fabricate a vertically oriented neutrondetector by depositing a first metal contact onto the first surfacecomprising a bottom surface of the at least one BN strip, and depositingthe second metal contact onto the second surface comprising a topsurface of the at least one BN strip.

The first metal contacts 606 ₁₋₃ and the second metal contacts 608 ₁₋₃can be deposited using an electron-beam evaporation, thermalevaporation, electroplating or pasting process. The thickness of the BNepilayers 602 ₁₋₃ can be from about several microns to greater than 300μm. The neutron detector 1000 can have one or more characteristics, suchas an operating voltage less than about 500 V, an operating temperaturebetween about −200 and 500 C, a gamma rejection ratio of about 1×10⁻⁶,or an energy resolution of about 3%. In some embodiments, the operatingvoltage less than about 100 V or the gamma rejection ratio is betterthan about 1×10⁻⁴.

It will be understood that particular embodiments described herein areshown by way of illustration and not as limitations of the invention.The principal features of this invention can be employed in variousembodiments without departing from the scope of the invention. Thoseskilled in the art will recognize, or be able to ascertain using no morethan routine experimentation, numerous equivalents to the specificprocedures described herein. Such equivalents are considered to bewithin the scope of this invention and are covered by the claims.

All publications and patent applications mentioned in the specificationare indicative of the level of skill of those skilled in the art towhich this invention pertains. All publications and patent applicationsare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

The use of the word “a” or “an” when used in conjunction with the term“comprising” in the claims and/or the specification may mean “one,” butit is also consistent with the meaning of “one or more,” “at least one,”and “one or more than one.” The use of the term “or” in the claims isused to mean “and/or” unless explicitly indicated to refer toalternatives only or the alternatives are mutually exclusive, althoughthe disclosure supports a definition that refers to only alternativesand “and/or.” Throughout this application, the term “about” is used toindicate that a value includes the inherent variation of error for thedevice, the method being employed to determine the value, or thevariation that exists among the study subjects.

As used in this specification and claim(s), the words “comprising” (andany form of comprising, such as “comprise” and “comprises”), “having”(and any form of having, such as “have” and “has”), “including” (and anyform of including, such as “includes” and “include”) or “containing”(and any form of containing, such as “contains” and “contain”) areinclusive or open-ended and do not exclude additional, unrecitedelements or method steps. In embodiments of any of the compositions andmethods provided herein, “comprising” may be replaced with “consistingessentially of” or “consisting of”. As used herein, the phrase“consisting essentially of” requires the specified integer(s) or stepsas well as those that do not materially affect the character or functionof the claimed invention. As used herein, the term “consisting” is usedto indicate the presence of the recited integer (e.g., a feature, anelement, a characteristic, a property, a method/process step or alimitation) or group of integers (e.g., feature(s), element(s),characteristic(s), property(ies), method/process steps or limitation(s))only.

The term “or combinations thereof” as used herein refers to allpermutations and combinations of the listed items preceding the term.For example, “A, B, C, or combinations thereof” is intended to includeat least one of: A, B, C, AB, AC, BC, or ABC, and if order is importantin a particular context, also BA, CA, CB, CBA, BCA, ACB, BAC, or CAB.Continuing with this example, expressly included are combinations thatcontain repeats of one or more item or term, such as BB, AAA, AB, BBC,AAABCCCC, CBBAAA, CABABB, and so forth. The skilled artisan willunderstand that typically there is no limit on the number of items orterms in any combination, unless otherwise apparent from the context.

As used herein, words of approximation such as, without limitation,“about”, “substantial” or “substantially” refers to a condition thatwhen so modified is understood to not necessarily be absolute or perfectbut would be considered close enough to those of ordinary skill in theart to warrant designating the condition as being present. The extent towhich the description may vary will depend on how great a change can beinstituted and still have one of ordinary skill in the art recognize themodified feature as still having the required characteristics andcapabilities of the unmodified feature. In general, but subject to thepreceding discussion, a numerical value herein that is modified by aword of approximation such as “about” may vary from the stated value byat least ±1, 2, 3, 4, 5, 6, 7, 10, 12 or 15%.

All of the compositions and/or methods disclosed and claimed herein canbe made and executed without undue experimentation in light of thepresent disclosure. While the compositions and methods of this inventionhave been described in terms of preferred embodiments, it will beapparent to those of skill in the art that variations may be applied tothe compositions and/or methods and in the steps or in the sequence ofsteps of the method described herein without departing from the concept,spirit and scope of the invention. All such similar substitutes andmodifications apparent to those skilled in the art are deemed to bewithin the spirit, scope and concept of the invention as defined by theappended claims.

To aid the Patent Office, and any readers of any patent issued on thisapplication in interpreting the claims appended hereto, applicants wishto note that they do not intend any of the appended claims to invokeparagraph 6 of 35 U.S.C. § 112, U.S.C. § 112 paragraph (f), orequivalent, as it exists on the date of filing hereof unless the words“means for” or “step for” are explicitly used in the particular claim.

For each of the claims, each dependent claim can depend both from theindependent claim and from each of the prior dependent claims for eachand every claim so long as the prior claim provides a proper antecedentbasis for a claim term or element.

REFERENCES

-   1. W. A. Noonan, Johns Hopkins Apl. Technical Digest 32, 762 (2014).-   2. J. Neal, L. Boatner, Z. Bell, H. Akkurt, and M. McCarthy,    “Evaluation of neutron and gamma detectors for high-temperature    well-logging applications,” in Future of Instrumentation    International Workshop (FIIW) (2011), pp. 172-175.-   3. I. Jun, I. Mitrofanov, M. L. Litvak, A. B. Sanin, W. Kim, A.    Behar, W. V. Boynton, L. DeFlores, F. Fedosov, D. Golovin, C.    Hardgrove, K. Harshman, A. S. Kozyrev, R. O. Kuzmin, A. Malakhov, M.    Mischna, J. Moersch, M. Mokrousov, S. Nikiforov, V. N. Shvetsov, C.    Tate, V. I. Tret'yakov, and A. Vostrukhin, J. Geophysical Research:    Planets 118, 11 (2013).-   4. G. F. Knoll, “Radiation detection and measurement,” 4th edition,    (John Wiley & Sons, 2010).-   5. Q. Shao, L. F. Voss, A. M. Conway, R. J. Nikolic, M. A. Dar,    and C. L. Cheung, Appl. Phys. Lett. 102, 063505 (2013).-   6. S. L. Bellinger, R. G. Fronk, W. J. McNeil, T. J. Sobering,    and D. S. McGregor, IEEE Trans. Nucl. Sci. 59, 167 (2012).-   7. S. L. Bellinger, R. G. Fronk, T. J. Sobering, and D. S. McGregor,    “High-efficiency microstructured semiconductor neutron detectors    that are arrayed, dual-integrated, and stacked,” Applied Radiation    and Isotopes 70, 1121 (2012).-   8. A. M. Conway, R. J. Nikolic, and T. F. Wang, Proceedings of the    International Semiconductor Device Research Conference, IEEE, New    York, pp. 589 (2007).-   9. Q. Shao, L. F. Voss, A. M. Conway, R. J. Nikolic, M. A. Dar,    and C. L. Cheung, Appl. Phys. Lett. 102, 063505 (2013).-   10. K. C Huang, R. Dahal, J. J. Q. Lu, A. Weltz, Y. Danon, and I. B.    Bhat, Nucl. Instrum. Methods Phys. Res. A 763, 260 (2014).-   11. U.S. Pat. No. 8,778,715B2, “Method of fabricating a neutron    detector such as a microstructured semiconductor neutron detector.”    Steven L. Bellinger, Ryan G. Fronk, Douglas S. McGregor.-   12. U.S. Pat. No. 7,164,138, “High-efficiency neutron detectors and    methods of making the same.” Douglas S. McGregor and Raymond Klann.-   13. U.S. Pat. No. 8,778,715, “Method of fabricating a neutron    detector such as a microstructured semiconductor neutron detector.”    Steven L. Bellinger, Ryan G. Fronk, and Douglas S. McGregor.-   14. U.S. Pat. No. 8,558,188, “Method for manufacturing solid-state    thermal neutron detectors with simultaneous high thermal neutron    detection efficiency (>50%) and neutron to gamma discrimination    (>1.0E4).” Rebecca J. Nikolic, Adam M. Conway, Daniel Heineck,    Lars F. Voss, Tzu Fang Wang, Qinghui Shao.-   15. U.S. Pat. No. 9,151,853B2, “Neutron-detecting apparatuses and    fabrication methods.” Rajendra P. Dahal, Jacky Kuan-Chih Huang,    James J. Q. Lu, Yaron Danon, and Ishwara B. Bhat-   16. U.S. Pat. No. 9,810,794, “Fabricating radiation-detecting    structures.” Rajendra P. Dahal, Ishwara B. Bhat, Yaron Danon, James    Jian-Qiang Lu.-   17. K. Osberg, N. Schemm, S. Balkir, J. O. Brand, M. S.    Hallbeck, P. A. Dowben, and M. W. Hoffman, IEEE Sensor J. 6 1531    (2006).-   18. Y. B. Losovyj, I. Ketsman, A. Sokolov, K. D. Belashchenko, P.    Dowben, J. Tang, and, Z. Wang, Appl. Phys. Lett 9, 1132908 (2007).-   19. D. S. McGregor, T. C. Unruh, and W. J. McNeil, Nucl. Instrum.    Methods Phys. Res. A 591, 530 (2008).-   20. A. N. Caruso, J. Physics: Condensed Matter 22, 443201 (2010).-   21. U.S. Pat. No. 6,727,504, “Boron nitride solid-state neutron    detector.” F. P. Doty.-   22. J. Uher, S. Pospisil, V. Linhart, and M. Schiebar, Appl. Phys.    Lett. 90, 124101 (2007).-   23. U.S. Pat. No. 9,093,581, “Structures and devices based on boron    nitride and boron nitride-III-nitride heterostructures.” Hongxing    Jiang, Sashi Majety, Rajedra Dahal, Jing L₁, and Jingyu Lin-   24. J. L₁, R. Dahal, S. Majety, J. Y. Lin, and H. X. Jiang, Nucl.    Instrum. Methods Phys. Res. A 654, 417 (2011).-   25. T. C. Doan, S. Majety, S. Grenadier, J. Li, J. Y. Lin, and H. X.    Jiang, Nucl. Instrum. Methods Phys. Res. A 748, 84 (2014); 783, 121    (2015).-   26. T. C. Doan, J. L₁, J. Y. Lin, and H. X. Jiang, AlP Advances 6,    075213 (2016).-   27. K. Ahmed, R. Dahal, A. Weltz, James J. Q. Lu, Y. Danon,    and I. B. Bhat, Appl. Phys. Lett. 110, 023503 (2017).-   28. A. Maity, T. C. Doan, J. L₁, J. Y. Lin, and H. X. Jiang, Appl.    Phys. Lett. 109, 072101 (2016).-   29. A. Maity, T. C. Doan, J. L₁, J. Y. Lin, and H. X. Jiang, Appl.    Phys. Lett. 111, 033507 (2017).-   30. A. Maity, S. J. Grenadier, J. L₁, J. Y. Lin, and H. X. Jiang, J.    Appl. Phys. 123, 044501 (2018).-   31. https://publishing.aip.org/publishing/j    ournal-highlights/hexagonal-boron-nitride-semiconductors-enable-cost-effective-detection-   32. H. G. Spieler and E. E. Haller, IEEE Trans. on Nucl. Sci.,    NS-32, 419 (1985).-   33. R. Dahal, K. Ahmed, J. Woei Wu, A. Weitz, J. Lu, Y. Danon    and I. B. Bhat, Applied Physics Express 9, 065801 (2016).-   34. X. K. Cao, B. Clubine, J. H. Edgar, J. Y. Lin, and H. X. Jiang,    Appl. Phys. Lett. 103, 191106 (2013).-   35. A. Many, J. Phys. Chem. Solids 26, 575 (1965).

What is claimed is:
 1. A neutron detector comprising: an electricallyinsulating submount; a BN epilayer of Boron-10 enriched hexagonal boronnitride (h-¹⁰BN or h-BN or ¹⁰BN or BN) placed on the insulatingsubmount; a first metal contact deposited on a first surface of the BNepilayer; and a second metal contact deposited on a second surface ofthe BN epilayer.
 2. The neutron detector of claim 1, further comprisinga measurement circuit connected to the first metal contact and thesecond metal contact.
 3. The neutron detector of claim 1, wherein thefirst surface comprises a side of the BN epilayer and the second surfacecomprises an opposite side of the BN epilayer such that the neutrondetector comprises a lateral oriented neutron detector.
 4. The neutrondetector of claim 3, wherein the first metal contact and the secondmetal contact each overlaps a top edge of the BN epilayer.
 5. Theneutron detector of claim 3, wherein: the BN epilayer comprises aplurality of BN strips placed end to end onto the electricallyinsulating submount in a series configuration; and each BN strip havingthe first metal contact and the second metal contact.
 6. The neutrondetector of claim 3, wherein: the BN epilayer comprises a plurality ofthe BN strips placed onto the electrically insulating submount in aparallel configuration; and each BN strip having the first metal contactand the second metal contact.
 7. The neutron detector of claim 1,wherein: the first metal contact is deposed on the first surfacecomprising a bottom surface of the BN epilayer; the second metal contactis disposed onto the second surface comprising a top surface of the BNepilayer; and the neutron detector comprises a vertical oriented neutrondetector.
 8. The neutron detector of claim 1, wherein the first metalcontact and the second metal contact are deposited using anelectron-beam evaporation, thermal evaporation, electroplating orpasting process.
 9. The neutron detector of claim 1, wherein the BNepilayer comprises at least two BN strips, each BN strip having a width(W) and at least two lengths (L₁, L₂).
 10. The neutron detector of claim1, wherein a thickness (t) of the BN epilayer comprises from severalmicrons to greater than 300 μm.
 11. The neutron detector of claim 1,wherein the neutron detector has one or more characteristics comprisingan operating voltage less than about 500 V, an operating temperaturebetween about −200 and 500 C, a gamma rejection ratio of about 1×10⁻⁴ orbetter, or an energy resolution of about 3%.
 12. The neutron detector ofclaim 1, wherein the neutron detector is integrated into a nuclearmaterial detection device, a geothermal and/or well logging device, or aplanetary mission device.
 13. The neutron detector of claim 1, whereinthe BN epilayer is flexible.
 14. A neutron detector comprising: anelectrically insulating submount; a BN epilayer of Boron-10 enrichedhexagonal boron nitride (h-¹⁰BN or h-BN or ¹⁰BN or BN) placed on theinsulating submount; a first metal contact deposited on a first surfaceof the BN epilayer, wherein the first contact surface comprises a side,an end or a bottom surface of the BN epilayer; a second metal contactdeposited on a second surface of the BN epilayer, wherein the secondcontact surface comprises an opposite side, an opposite end or a topsurface of the BN epilayer; a measurement circuit connected to the firstmetal contact and the second metal contact.
 15. The neutron detector ofclaim 14, wherein the BN epilayer comprises at least two BN strips, eachBN strip having a width (W) and at least two lengths (L₁, L₂).
 16. Theneutron detector of claim 14, wherein a thickness (t) of the BN epilayercomprises from several microns to greater than 300 μm.
 17. The neutrondetector of claim 14, wherein the neutron detector has one or morecharacteristics comprising an operating voltage less than about 500 V,an operating temperature between about −200 and 500 C, a gamma rejectionratio of about 1×10⁻⁴ or better, or an energy resolution of about 3%.18. The neutron detector of claim 14, wherein the neutron detector isintegrated into a nuclear material detection device, a geothermal and/orwell logging device, or a planetary mission device.
 19. The neutrondetector of claim 14, wherein the BN epilayer is flexible.
 20. A neutrondetector fabricated by a process comprising the steps of: providing anepilayer wafer of Boron-10 enriched hexagonal boron nitride (h-¹⁰BN orh-BN or ¹⁰BN or BN) having a thickness (t); dicing or cutting theepilayer wafer into one or more BN strips having a width (W) and alength (L); and depositing a first metal contact on a first surface ofat least one of the BN strips and a second metal contact on a secondsurface of the at least one BN strip.
 21. The neutron detector by theprocess of claim 20, further comprising connecting the first metalcontact and the second metal contact to a measurement circuit.
 22. Theneutron detector by the process of claim 21, further comprisingmeasuring a plurality of neutrons using the neutron detector.
 23. Theneutron detector by the process of claim 22, further comprising usingthe neutron detector to detect nuclear materials, to perform geothermaland/or well logging, or perform planetary missions.
 24. The neutrondetector by the process of claim 20, further comprising: placing the atleast one BN strip onto an electrically insulating submount prior todepositing the first metal contact and the second metal contact; andwherein the first surface comprises a side of the at least one BN stripand the second surface comprises an opposite side of the at least one BNstrip such that the neutron detector comprises a lateral orientedneutron detector.
 25. The neutron detector by the process of claim 24,wherein the first metal contact and the second metal contact eachoverlaps a top edge of the at least one BN strip.
 26. The neutrondetector by the process of claim 24, further comprising repeating theplacing step and depositing step such that a plurality of BN strips areplaced end to end onto the electrically insulating submount in a seriesconfiguration.
 27. The neutron detector by the process of claim 24,further comprising repeating the placing step and depositing step suchthat a plurality of the neutron detectors are placed onto theelectrically insulating submount in a parallel configuration.
 28. Theneutron detector by the process of claim 20, wherein depositing thefirst metal contact on the first surface of the at least one BN stripand the second metal contact on the second surface of the at least oneBN strip comprises: depositing a first metal contact onto the firstsurface comprising a bottom surface of the at least one BN strip; anddepositing the second metal contact onto the second surface comprising atop surface of the at least one BN strip such that the neutron detectorcomprises a vertical oriented neutron detector.
 29. The neutron detectorby the process of claim 20, wherein depositing the first metal contacton the first surface of the at least one BN strip and the second metalcontact on the second surface of the at least one BN strip is preformedusing an electron-beam evaporation, thermal evaporation, electroplatingor pasting process.
 30. The neutron detector by the process of claim 20,wherein the one or more BN strips comprise at least two strips, eachstrip having the width (W) and at least two lengths (L₁, L₂).
 31. Theneutron detector by the process of claim 20, wherein the thickness ofthe epilayer wafer comprises several microns to greater than 300 μm. 32.The neutron detector by the process of claim 20, wherein providing theepilayer wafer comprises: providing a substrate; growing the epilayerwafer on the substrate; and removing the epilayer wafer from thesubstrate.
 33. The neutron detector by the process of claim 32, whereinremoving the epilayer wafer from the substrate comprises cooling theepilayer wafer and the substrate such that a difference in thermalexpansion coefficients between the epilayer wafer and the substrateautomatically separates the epilayer wafer from the substrate.
 34. Theneutron detector by the process of claim 20, wherein the epilayer waferis flexible.
 35. The neutron detector by the process of claim 20,wherein the neutron detector has one or more characteristics comprisingan operating voltage less than about 500 V, an operating temperaturebetween about −200 and 500 C, a gamma rejection ratio of about 1×10⁻⁴ orbetter, or an energy resolution of about 3%.